The present invention relates to a bias-voltage generator, and more particularly to a bias-voltage generator producing a bias voltage to be applied to a semiconductor substrate on which a MOS (Metal Oxide Semiconductor) integrated circuit is mounted.
Generally, the MOS integrated circuit is provided with the bias-voltage generator so as to establish and maintain the desired optimum circuit operation of each of the semiconductor devices, such as FETs (Field Effect Transistors), which constitute the MOS integrated circuit. For instance, if the threshold voltage is below a specified or nominal level, the semiconductor device is unstable in operation, as it may be improperly rendered conductive by a noise signal, and is thus particularly unsuitable for use in logic circuits. However, if the threshold voltage is greater than the desired level, the speed of the operation of the semiconductor device is reduced, and the semiconductor device may fail to be turned on when an input signal is applied to the control electrode. Thus, the level of the threshold voltage must be maintained at a desired optimum level. The maintenance of the level of the threshold voltage at a desired optimum level is achieved by the bias-voltage generator, whose function is to absorb the so-called substrate current which flows through the substrate.
In the prior art, such a bias-voltage generator has been proposed in, for example U.S. Pat. No. 3,806,741, entitled SELF-BIASING TECHNIQUE FOR MOS SUBSTRATE VOLTAGE, by Frederick J. Smith. In this technique the bias-voltage generator can absorb the substrate current in such a manner that the amplitude of the current varies in accordance with the variation of the operating frequency of a dynamic-type circuit, for example a dynamic-type storage device in which there is a lower limit which respect to the operating frequency. This is because such a dynamic-type storage device is operated by using electric charges stored in the stray capacitances.
Briefly speaking, in the above mentioned prior art bias-voltage generator, the output signal to be produced from the bias-voltage generator and supplied to the substrate, varies in magnitude of the voltage thereof in accordance with the variation of the operating frequency of the dynamic-type circuit.
Contrary to the above, in the present invention, as will be explained hereinafter, the output signal to be produced from the bias-voltage generator and supplied to the substrate, varies in frequency in accordance with the variation of the voltage level of the semiconductor substrate.
In recent years, a great number of MOS devices, for fabricating a desired MOS integrated circuit, have been mounted on a single semiconductor chip with a very high degree of integration. Accordingly, in each MOS device, the channel between the drain and the source thereof should considerably be shortened in length. Thus, a very short channel must be formed therein. However, as the length of the channel becomes shorter, the intensity of the electric field created adjacent to the drain becomes stronger. As a result, an impact ionization current is induced across the electric field. At this time, separated holes and electrons move in respective directions. That is, the electrons move toward the drain, while the holes move toward the substrate. These separated holes and electrons create a bad effect on the normal operation of a MOS integrated circuit. In particular, the holes absorbed into the substrate create a serious problem. This is because the voltage level of the substrate, with respect to a ground of a MOS integrated circuit, is unnecessarily increased by the holes themselves, from the optimum voltage level.